[CodeGen] remove instcombine from codegen tests; NFC

[CodeGen] remove instcombine from codegen tests; NFC

The FileCheck lines in these files are auto-generated and complete,
so there's very little upside (less CHECK lines) from running
-instcombine on them and violating the expected test layering
(optimizer developers shouldn't have to be aware of clang tests).

Running opt passes like this makes it harder to make changes such as:
D93817